In order to produce integrated circuits, layers with different electrical characteristics are normally applied to semiconductor wafers, and are structured lithographically. A lithographic structuring step may include applying a photosensitive resist, exposing and developing of the resist with a predetermined structure for the relevant plane, and transferring the resist mask to the layer underneath in an etching step.
A scanner or stepper is normally used as the exposure apparatus for the lithographic projection step for a circuit pattern. In the exposure apparatus, the photosensitive resist is exposed to electromagnetic radiation at a predetermined wavelength, which, for example, is in the UV band. The exposure dose produced by the exposure of the resist at the location of the semiconductor wafer is chosen in accordance with the specifications for the resist layer. The mean dose required for structure mapping is typically approximately 30 mJ/cm2.
Each individual layer of the circuit pattern is normally mapped onto the semiconductor wafer by a mask (or reticle) and projection optics. The reticle includes a substrate layer, which is provided with absorbent elements, such as a chromium layer, which model the circuit pattern. The reticle is generally provided with a protective film, i.e., the pellicle. The pellicle is used to protect the structure face against deposits. The projection optics in the exposure apparatus frequently result in a reduction of the circuit pattern during the transfer to the resist.
The semiconductor wafer is generally placed on a substrate holder and is moved to an appropriate position for exposure. The circuit pattern arranged on a mask is then successively transferred to individual exposure fields on the photosensitive resist. The size of an exposure field is normally about 25 mm×35 mm.
The reticles, which are used in the lithographic exposure process, are subject to mechanical loads, which may cause defects or contamination, as a result of the movements within and outside the exposure systems.
Furthermore, particles and contamination may become attached to the surface by adhesion from the surrounding atmosphere, so that the reticles must be monitored within predetermined time intervals, but at least before use after a lengthy pause in use.
In order to allow such monitoring to be carried out in a large-volume manufacturing process and with a wide range of products, the number of reticle movements are generally counted automatically. In this case, the number of movements of the reticle within and outside the exposure apparatuses, for example, with respect to a storage location, are normally counted, and a monitoring limit is derived from this value. Furthermore, a rigid time schedule is predetermined, determining a further monitoring limit as a function of the period of use. Reticle monitoring is then carried out upon reaching the rigid monitoring limits, during which process macro inspections or defect inspections are carried out.
Macro inspections are large-area oblique light inspections in white light in order to identify defect locations or particles above a size of about 10 μm in scattered light. In order to identify smaller defects, other methods are used, for example, laser beam scanning methods, scatterometry, or reverse image identification with layer comparison.
One problem which has not been observed much until now in this context is that the monitoring limits are defined independently of the actual radiation load on the reticle. As a result of the structure transfer of the structures on the reticle to the semiconductor wafer through reduction optics of electromagnetic radiation, the reticle is subject to a not inconsiderable radiation load, which leads to damage and contamination as a result of energy absorption and photochemical processes in the various layers of the reticle material. A rigid monitoring system ignores the fact that observed recrystallization on the reticle front face and rear face as well as pellicle cloudiness, which may occur, may be initiated by the influence of electromagnetic radiation.
Thus, for example, the presence of ammonium ions and carbon dioxide on the reticle surface leads to the formation of tricyanic acid crystals or ammonium and sulfate ions in order to form ammonium sulfate, both of which can grow when illuminated with energy, depending on the wavelength. The presence of ammonium ions and carbon dioxide leads to the formation of ammonium acid crystals on the reticle surface, which may grow when illuminated with energy, depending on the wavelengths.